Vehicular fuel level senders, as they are commonly called, are actually transducers that convert the level of fuel in the fuel tank to an electric signal to proportionally drive a visually readable electric instrument such as a bitorque gauge. These fuel senders conventionally include a float caried on the end of a long pivoting arm suspended within the interior of the fuel supply tank, and this arm pivots a short wiper blade having a ball contact slidably engaging the wires on a wound wire rheostat that varies in resistance in accordance with the position of the float and hence liquid level, however not always in directly proportional fashion. The angular position of the pivoting float arm, or more particularly the incremental angle of the arm is not directly proportional to vertical incremental change in the float position and hence not directly proportional to liquid level. Moreover, in some cases the instrument itself is not proportional throughout its range and requires some compensation, and in other cases the tank itself may be irregular in configuration.
The rheostat, and more particularly the wire conductive winding on the rheostat, provides a convenient location for compensating for these non-linearities usually in one of two, or both, ways. The first is to vary the spacing between the wire turns and more specifically by increasing wire turn spacing the ratio of resistance change to float arm angle change decreases, and conversely it increases with more tightly wound turns. Another common way of varying linearity is to change the individual turn lengths by varying the shape of the support board on which the wire is wound. One common shape variation includes a tapered section in the form of a frusto-isosceles triangle.
This differential wire turn spacing as well as the wire turn length variation has created a problem because the wiper contact as it rides across the wire turns tends to pluck or "banjo" the loosely-held wires and not infrequently causes rupture of the wire, which of course results in a complete failure in the rheostat.
One way to minimize this wire banjoing effect is to glue the wires to the board with epoxy by applying an epoxy adhesive to the board, then wind the wire over the pre-coated board, and thereafter cure the epoxy in an oven. This is an extremely difficult process to control, particularly with respect to limiting the areas to which glue is applied, and of course the epoxy around the wire windings can insulate the wire and destroy the electrical output of the rheostat. Moreover this is an extremely costly method of manufacture.
Another attempt at solving this wire plucking problem has been to mold projections on the rheostat support board similar to the bridge of a stringed musical instrument; however, the bridges tend to stretch the wire windings so that they are over-tensioned which actually increases the likelihood of rupture. It has also been found difficult to reliably wind the wire on these rather exacting projections.
It is a primary object of the present invention to ameliorate the problems noted above with rheostats in fuel level sender assemblies.